1. Field of the Invention
This invention relates to semi-aqueous stripping and cleaning compositions that are particularly useful for stripping photoresists and cleaning organic and inorganic compounds, including post etch and post ash residues, from a semiconductor substrate. The invention is also useful as a silicon oxide etchant. As used herein, the term xe2x80x9csemi aqueousxe2x80x9d refers to a mixture of water and organic solvent. The invention also includes methods of using this composition to strip photoresists, clean organic and inorganic compounds from semiconductor substrates, and etch silicon oxide. More particularly, the invention describes semi aqueous stripping, cleaning and etching compositions and processes for the use thereof. The solutions contain fluoride compounds, sulfoxide or sulfone solvents, water and may contain other solvents, corrosion inhibitors, chelating agents, basic amine compounds, surfactants, acids and bases.
2. Description of Related Art
Fluoride containing chemistries have been used for many years with prime silicon wafers (wafers that have not yet undergone ion implantation or device construction) in the semiconductor industry. Normally the fluoride chemistry (usually dilute hydrofluoric acid) is used as the last process step in the sequence called xe2x80x9cRCA rinsesxe2x80x9d. The substrate is often contaminated from previous process steps with monolayer amounts of metal, anions and/or organic contaminants or surface residues (particles). These contaminants have been shown to have a significant impact on the electrical integrity of simple test device structures and these structures need to be cleaned efficiently without impairing their integrity. Such cleaning methods could include techniques discussed in the technical literature, for example, mt. Conf. On Solid State Devices and Materials, 1991, pp. 484-486 or Kujime, T. et al., Proc. of the 1996 Semi. Pure Water and Chemicals, pp. 245-256 and Singer, P. Semi. International, p. 88, October 1995.
Patents that teach methods for cleaning prime wafers with low pH solutions include U.S. Pat. Nos. 5,560,857 and 5,645,737; 5,181,985; 5,603,849; 5,705,089.
Using fluoride chemistries (usually HF) as a final RCA cleaning step will cause the silicon wafer surface to be in a hydrophobic state (the surface is covered with Sixe2x80x94H groups) which will repel water. During this step a certain proportion of the wafer surface is dissolved (removed). Unless the conditions are carefully monitored (time, temperature, solution composition) the substrates can be damaged, as reported by Rafols, C. et al., J. Electroanalytic Chem. 433. pp. 77-83, 1997. Numerous compositions combine water and organic solvents. The water concentration in these solutions is very critical. Silica oxide has an etch rate of 21 xc3x85/min (@25xc2x0 C.) in HF/water, but in isobutanol the rate was reduced to 2.14 xc3x85/min and even lower in acetone (an aprotic solvent) the rate was only 0.12 xc3x85/min, as reported at NSF/SRC Eng. Res. Center, Environmentally Benign Semiconductor Manufacturing, Aug. 5-7, 1998, Stanford University.
After the Front End of Line (FEOL) cleaning process the wafer proceeds to the typical Back End of Line (BEOL) manufacturing process for a semiconductor devices, in which the devices might be dynamic random access memories (DRAMs), static random access memories (SRAMs), logic, electrically programmable read only memories (EPROMs), complementary metal on silicon (CMOS), and the like. Etching fabrication technology using chemical reactions (liquid or plasma) has been used as a method of forming a wiring structure on such semiconductor substrates.
A photoresist film is deposited on the wafer to form a mask, then a substrate design is imaged on the film layer, baked, and the undeveloped image is removed with a developer. The remaining image is then transferred to the underlying material through etching (either a dielectric or metal) with reactive etching gases promoted with plasma energy.
The etchant gases selectively attack the unprotected area of the substrate. Liquid or wet etching chemistries have been used extensively over the years to etch metals, oxides and dielectrics. These chemistries can be very aggressive and can result in isotropic etching (etching equally in all directions).
Increasingly, plasma etching, reactive ion etching or ion milling are used, and such etching processes produce undesirable by-products from the interaction of the plasma gases, reacted species and the photoresist. The composition of such by-products is generally made up of the etched substrates, underlying substrate, photoresist and etching gases. The formation of such by-products is influenced by the type of etching equipment, process conditions and substrates utilized. These by-products are generally referred to as xe2x80x9csidewall polymer,xe2x80x9d xe2x80x9cveilxe2x80x9d or xe2x80x9cfencesxe2x80x9d and cannot be removed completely by either oxygen plasma or conventional solvents. Examples of alkaline/solvent mixture types of photoresist strippers which are known for use in stripping applications include dimethylacetamide or dimethylformamide and alkanolamines as described in U.S. Pat. Nos. 4,770,713 and 4,403,029; 2-pyrrolidone, dialkylsulfone and alkanolamines as described in U.S. Pat. Nos. 4,428,871, 4,401,747, and 4,395,479; and 2-pyrrolidone and tetramethylammonium hydroxide as described in U.S. Pat. No. 4,744,834. Such stripping compositions, however, have only proven successful in cleaning xe2x80x9csidewall polymerxe2x80x9d from the contact openings and metal line etching in simple microcircuit manufacturing involving a single layer of metal process when the metal structure involves mainly Alxe2x80x94Si or Alxe2x80x94Sixe2x80x94Cu and the xe2x80x9csidewall polymerxe2x80x9d residue contains only an organometallic compound with aluminum.
If etching residue is not removed from the substrate, the residue can interfere with subsequent processes involving the substrate. The need to effectively remove etching residue and photoresist from a substrate becomes more critical as the industry progresses into submicron processing techniques. The requirement for cleaning solutions that remove all types of residue generated as a result of plasma etching of various types of metals, such as aluminum, aluminum/silicon/copper, titanium, titanium nitride, titanium/tungsten, tungsten, silicon oxide, polysilicon crystal, etc., while not corroding the underlying metal presents a need for more effective chemistry in the processing area. The effect of poor cleaning results in low device yield, low device reliability, and low device performance.
Also, if the components in these residues are not removed or neutralized in some manner then the residues will absorb moisture and form acidic species that can corrode the metal structures. The resultant acid corrodes wiring materials to bring about an adverse effect such as an increase in electrical resistance and wire disconnection. Such problems frequently occur, in particular in aluminum and aluminum alloys generally used as wiring material. The wafer substrate in contact with acidic materials, if not controlled, can destroy the metal structures. Following completion of the etching operation it is necessary that the post-etch resist mask be removed from the protective surface to permit finishing operations.
It is desirable to develop an improved cleaning composition to remove the organic polymeric substance from a coated inorganic substrate without corroding, dissolving or dulling the metal circuitry or chemically altering the wafer substrate.
Sidewall residues have been removed with either acidic organic solvents or alkaline organic solvents. The acidic solvents are generally composed of phenolic compounds or chloro-solvent and/or an aromatic hydrocarbon and/or alkylbenzenesulfonic acids. These formulations generally need to be used at temperatures up to and beyond 100xc2x0 C. These chemistries normally need to be rinsed with isopropanol.
In addition, stripping compositions used for removing photoresist coatings and cleaning composition for removing post-etch residue have for the most part been highly flammable, generally hazardous to both humans and the environment, and comprise reactive solvent mixtures exhibiting an undesirable degree of toxicity. Moreover, these compositions are not only toxic, but their disposal is costly since they might have to be disposed of as a hazardous waste. In addition, these compositions generally have severely limited bath life and, for the most part, are not recyclable or reusable.
The photoresist around the contact hole of common interlayer dielectrics, TEOS (tetraethylorthosilicate) and boron phosphosilicate glass (BPSG), which are commonly used in ultra large scale integration (ULSI) structures for better conformity of step coverage, is usually removed with HF solutions. It is not uncommon for the HF to also attack the dielectric material. Such attack is not desirable (see Lee, C. and Lee, 5, Solid State Electronics, 4, pp. 921-923 (1997)). Accordingly, a need exists for a more environmentally friendly stripping and cleaning formulation.
Dilute hydrofluoric acid solutions can under certain conditions remove the sidewall polymers by aggressively attacking the via sidewall of the dielectric and therefore changing the dimensions of the device, as taught by Ireland, P., Thin Solid Films, 304, pp. 1-12 (1997), and possibly the dielectric constant. Previous chemistries that contain HF, nitric acid, water and hydroxylamine are aggressive enough to etch silicon, as taught by U.S. Pat. No. 3,592,773 issued to A. Muller. Recent information also indicates that the dilute HF solutions can be ineffective for cleaning the newer CFx etch residues, as taught by K. Ueno et al., xe2x80x9cCleaning of CHF3 Plasma-Etched SiO2/SiN/Cu Via Structures with Dilute Hydrofluoric Acid Solutions,xe2x80x9d J. Electrochem. Soc., vol. 144, (7) 1997. Contact holes opened on to the TiSi2 have also been difficult to clean with HF solutions since there appears to be an attack of the underlying TiSi2 layer. There may also be difficulty with mass transport of the chemicals in the narrow hydrophilic contact holes, as taught by Baklanov, M. R. et al., Proc. Electrochem. Soc., 1998, 97-35, pp. 602-609.
Recently, fluoride-based chemistries have been used in limited cases to remove post etch residues. Many of these compositions contain fluoride components, specifically hydrogen fluoride. In addition these compositions might contain strong caustic chemicals (choline-derivatives, tetraalkyl ammonium hydroxide, ammonium hydroxide) such as disclosed in U.S. Pat. No. 5,129,955; U.S. Pat. No. 5,563,119; or U.S. Pat. No. 5,571,447, or might use a two-phase solvent system, which contains one phase with hydrofluoric acid and water while a second phase contains a nonpolar organic solvent (ketones, ethers, alkanes or alkenes) (U.S. Pat. No. 5,603,849). Other formulations include hydroxylamine and ammonium fluoride (U.S. Pat. No. 5,709,756, issued to Ward). Additional examples include quaternary ammonium salt and fluoride based compositions, as disclosed in published European Application 0662705, and organocarboxylic ammonium salt or amine carboxylate and fluoride based compositions, as disclosed in U.S. Pat. No. 5,630,904.
Other methods for cleaning metal and metal oxide residues on wafers include spraying water vapor into the plasma ashing chamber followed by introducing fluorine containing gases (hydrofluoric acid) (U.S. Pat. No. 5,181,985) or a liquid containing hydrofluoric acid. ammonium fluoride and water with a pH between 1.5 to less than 7.
Some chemistries have also included chelating agents to help remove ionic and anionic contamination from the wafer surface (PCT US98/02794) but chelating agents such as citric acid, gallic acid, and catechol among others, can be aggressive toward the aluminum oxide that covers the Al metal lines. Studies by Ohman and Sjoberg show that the strong complexing ability of citric ions can increase the aluminum oxide solubility and thereby expose the metal to further corrosion, by factors of 166 and 468 at pH 5 and 6 (see Ohman et al., J. Chem. Soc., Dalton Trans. (1983), p. 2513).
Other resist-remover chemistries, such as those in U.S. Pat. No. 5,792,274, have included a salt of hydrogen fluoride combined with a water-soluble organic solvent and water at a pH of 5 to 8. However, no mention is made of the use of ammonium hydrogen fluoride (or ammonium bifluoride), which provides greater stability than ammonium fluoride, or the use of a synergistic mixture of co-solvents or basic amine compounds with DMSO and a fluorinated compound.
U.S. Pat. No. 6,048,406 issued Apr. 11, 2000 to Misra et al. entitled xe2x80x9cBenign Method for Etching Silicon Dioxidexe2x80x9d teaches using an aqueous solution of ammonium hydrogen biflouride ((NH4)HF2) as an alternative to hydrofluoric acid because it is more benign for wet etching silicon oxide. However, there is no teaching of the use of a formulation that can remove photoresist or an etching residue. Also, there is no teaching of adding a synergistic mixture of co-solvents or basic amine compounds.
U.S. Pat. No. 5,885,477 issued Mar. 23, 1999 to Rasmussen et al. entitled xe2x80x9cSilicon Dioxide Etch Process Which Protects Metalxe2x80x9d teaches using an aqueous solution of ammonium fluoride and hydrofluoric acid with a salt to etch silicon oxide while minimizing corrosion. However, there is no teaching of the use of ammonium hydrogen bifluoride, co-solvents or basic amine compounds.
U.S. Pat. No. 4,508,591 issued Apr. 2, 1985 to Bartlett et al. entitled xe2x80x9cPolymethyl Methacrylate Compatible Silicon Dioxide Complexing Agentxe2x80x9d teaches using ammonium fluoride and citric acid to etch silicon dioxide. However, as with Rasmussen et al., there is no teaching of the use of ammonium hydrogen bifluoride, co-solvents or basic amine compounds. Nor is there any teaching to use such a formulation for removing etch residue or photoresist.
Accordingly, there exists a need to develop improved silicon dioxide etchant and photoresist and post-etch residue remover for a variety of unwanted materials from a wide variety of substrates. Particularly in the field of integrated circuit fabrication, it should be recognized that the demands for improved removal performance with avoidance of attack on the substrates are constantly increasing. This means that compositions that were suitable for less sophisticated integrated circuit substrates may not be able to produce satisfactory results with substrates containing more advanced integrated circuits in the process of fabrication. These compositions should also be economical, environmental friendly and easy to use.
The present invention teaches such a new and improved stripping and cleaning composition and a process for its use. The present invention also includes silicon oxide etching compositions and their use. This composition is aqueous, dissolves both organic and inorganic substances, and, when used in the process, is able to strip and clean a variety of substrates.
The novel stripping, cleaning and etching compositions of the invention exhibit synergistically enhanced capabilities that are not possible from the use of the individual components, or the components in combination with other components.
It is one objective of the invention to provide etch residue removing compositions that: effectively clean post-etch residues from substrates, inhibit redeposition of metal ions, and are corrosion resistant.
It is a further objective of the invention to provide effective photoresist stripping compositions.
Another objective is to provide effective silicon oxide etching compositions.
These and related objectives are attained through the use of the composition and process disclosed herein.
A composition in accordance with this invention is for photoresist stripping and comprises from about 0.01 percent by weight to about 10 percent by weight of one or more fluoride compounds, an effective amount of up to about 95 percent by weight of one or more solvents that are sulfoxides or sulfones, and at least about 20 percent by weight water. Additionally, the composition may optionally contain basic amines, co-solvents, corrosion inhibitors, chelating agents, surfactants, acids and bases. A preferred embodiment contains ammonium hydrogen fluoride, DMSO, and water.
A composition in accordance with this invention is for post-etch residue cleaning and comprises from about 0.01 percent by weight to about 10 percent by weight of one or ore fluoride compounds, an effective amount of from about 10 to about 95 percent by eight of one or more certain solvents, and at least about 20 percent by weight water. A referred embodiment consists of ammonium hydrogen fluoride, DMSO, and water.
Additionally, the composition may optionally contain basic amines, co-solvents, corrosion inhibitors, chelating agents, surfactants, acids and bases.
Another preferred composition for cleaning and stripping consists of ammonium hydrogen fluoride, DMSO, water, and a co-solvent selected from the group consisting of acetic acid, methyl acetate, methyl lactate, ethyl acetate, ethylene glycol diacetate, ethyl lactate, propylene glycol, propylene carbonate, N-methyl pyrrolidone, methoxyethoxyethanol and polyethylene glycol monolaurate.
Another preferred composition for cleaning and stripping consists of ammonium hydrogen fluoride, DMSO, water, and a basic amine selected from the group consisting of hydroxylamine, hydrazine, 2-amino-2-ethoxy ethanol, monoethanolamine, diethylhydroxylamine, choline, tetramethylammonium formate, monoisopropanolamine, diethanolamine, and triethanolamine.
If said composition in accordance with this invention is used for metal and oxide etching, the content of the fluoride compound is elevated. For example, for use as a silicon etchant, it is preferred to have about 20% of a 40% aqueous ammonium fluoride solution.
A process for photoresist stripping in accordance with this invention comprises contacting the substrate with a composition comprising one or more fluoride compounds, water and sulfoxide solvent at a temperature and for a time sufficient to strip the photoresist.
A process for cleaning residue from a substrate in accordance with this invention comprises contacting the substrate with a composition comprising one or more fluoride compounds, water and sulfoxide solvent at a temperature and for a time sufficient to clean the substrate.
A process for metal or oxide etch in accordance with this invention comprises contacting the metal or oxide with a composition comprising one or more fluoride compounds, water and sulfoxide or sulfone solvent at a temperature and for a time sufficient to etch the metal or oxide.